Chromium catalysts, sometimes termed Phillips catalysts, are well known catalysts for olefin polymerization. In these catalysts, a chromium compound, such as chromium oxide, is supported on a support of one or more inorganic oxides such as silica, alumina, zirconia or thoria, and activated by heating in a non-reducing atmosphere. U.S. Pat. No. 2,825,721 describes chromium catalysts and methods of making the catalysts. It is also known to increase polymer melt index by using a silica-titania support as disclosed, for example, in U.S. Pat. No. 3,887,494.
European patent application EP 1 172 381 A1 discloses a method for producing ethylene polymers reputedly having improved environmental stress crack resistance (ESCR) using a trialkylaluminum compound-carried chromium catalyst. The activated chromium compound on an inorganic support is treated with a trialkylaluminum compound in an inert hydrocarbon solvent and the solvent removed to form a trialkylaluminum-carried chromium catalyst. EP 1 172 381 A1 discloses that the time of contact with the solvent must be minimized to avoid over-reduction and associated degradation of polymer properties. The reference further teaches that for ESCR and impact resistance it is essential to use hydrogen in a carefully controlled hydrogen-ethylene ratio. The need for careful control of contact time, hydrogen concentration, and amount of trialkylaluminum used in the catalyst synthesis in order to achieve desired polymer properties limits the usefulness of such methods.
U.S. Pat. No. 6,174,981 discloses a process of polymerizing ethylene and at least one C3 to C8 mono-olefin in the presence of a catalyst including chromium supported on a silica-titania support and a trialkylboron compound. The process, however, produces resins that generally have density and HLMI (high load melt index, I21.6) values lower than would be desirable for environmentally demanding applications such as for pipe or drum resins.
Other relevant references include EP 0279890; WO 01 32307; DE 2622755; and U.S. Pat. No. 4,877,763.